All electrical conductors have capacitive and inductive properties and as a result, are capable of storing energy. If the insulating barrier between two conductors breaks down, a discharge path for the stored energy is formed, and a stored energy discharge along this path creates an "arc". If the energy discharge or "arc" exceeds the wattage rating of the passive components or of the solid state circuitry of the electrical equipment, the arc can damage the instrument.
An arc creates a low resistance path, thus, providing a path for the continual discharge of energy. The presence of in-line power supplies operate to sustain the arc condition by providing current to the arc path. The presence of the arc current creates a current spike which, in turn, generates an associated electromagnetic field spike.
Arcs are inherent in the design and operation of many electrical instruments and thus, cannot be totally eliminated. The presence of a high voltage in the instrument further complicates attempts at arc suppression since the voltage parameters associated with the high instrument voltage often exceed the ratings of potential high speed, solid state circuit breakers. In the past, this problem of arc suppression in high voltage instruments required the use of mechanical circuit breakers which did not have the sensitivity or the response time required to protect the instrument against damage from the arc.
Applicant's invention senses the presence of an arc by monitoring the strength of the electromagnetic field surrounding a conductor whose current and electromagnetic field are effected by the presence of the arc. The apparatus monitors the electromagnetic field through the use of a Hall-Effect transducer. When exposed to an electromagnetic field, the Hall-Effect transducer experiences an induced voltage which is proportional to the strength of the electromagnetic field. Since the onset of an arc results in a change in the electromagnetic field surrounding the conductor, the transducer experiences a change in the induced voltage in response to the arc. Therefore, by using a Hall-Effect transducer, one can correlate a direct relationship between the transducer's output voltage and the magnitude of the current surge caused by the arc.
In Applicant's invention, the output voltage from a Hall-Effect transducer is further conditioned and amplified, and then, this modified transducer signal is compared to a selected threshold value to determine whether or not the arc poses a danger to the instrument. If the are exceeds the threshold value, a circuit breaker is engaged which shunts the arc-effected signal to ground. Input over-voltage protection is provided to the transducer through the use of back-to-back clamp diodes, and gain adjustment circuitry is employed to allow for adjustment in transducer sensitivity.
The above configuration employs hysteresis feedback techniques in sensing the condition of the circuitry and employs arc suppression delay filtering to detect arc propagation rates as fast as 100 nanoseconds with an impressed voltage of up to 10 KV superimposed on a dc voltage. By employing Applicant's system, the switching speed of the circuit breaker can be reduced from the millisecond range of the standard mechanical circuit breakers currently in use to microseconds.
This invention is useful because it requires only minor modification of the circuit. The invention simply clamps on an active circuit of the instrument and only requires the insertion of the arc suppression circuit, a switch and appropriate ground cables.
It is the objective of the present invention to provide arc protection to arc sensitive instrumentation operating at high instrument voltages. Another object of this invention is to provide for a variable threshold which must be exceeded by the strength of the arc in order to activate the high speed circuit breaker. It is, also, the object of the invention to provide for the rapid cycling of the high speed circuit breaker to minimize the off-line time of the instrument.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of instrumentalities and combinations particularly pointed out in the appended claims.